I used a create on a need-to-have basis approach. Start with some general “Discussion” channel, as recurring & common topics emerge create separate channels if you see a benefit from such separation. I guess it’s not that much different from how you’d start structuring the forum part of the discourse.
Also, I’ve enabled the chat for tl3 & tl4 for some time and gradually started giving access to tl2, tl1 and tl0. The users will follow whatever the rules and norms are established when they join the chat.
I guess it does depend on the specific topic of those communities. We didn’t mirror the categories because a) we didn’t see a clear reason why we’d benefit from it b) it underlines the notion that chat is not just another place to hold discussions that were previously contained within forum topics.